An acoustic exciter is made by combining a magnetic circuit and a vibrator with a suspension having a spring property. Vibration is generated as the result of transaction between the magnetic circuit and the vibrator attracting/repelling to each other. The vibration is conducted to a vibration staff on which the acoustic exciter is mounted. A conventional acoustic exciter is described below referring to FIG. 8 which shows a cross sectional side view of acoustic exciter and FIG. 9 which shows an equivalent circuit diagram representing its mechanical system.
As shown in FIG. 8, a conventional acoustic exciter is formed of magnetic circuit 221 and vibrator 226. Magnetic circuit 221 includes yoke 222, magnet 223 and plate 224, and provides magnetic gap 221a. The magnetic circuit is connected to suspension 225 which is made of an elastic plate material. Vibrator 226 is formed of vibrating section 227, voice coil 228 connected to vibrating section 227, and frame section 229 which connects vibrating section 227 with suspension 225.
Vibrating section 227 and frame section 229 are integrally formed as a unitized body by means of resin molding.
When electricity is led to voice coil 228 of the above-structured acoustic exciter, attracting/repelling forces are generated with respect to magnetic circuit 221. Vibrator 226 and magnetic circuit 221 start vibrating, which vibration excites a vibration staff (not shown) connected to vibrating section 227. An acoustic exciter makes vibration staff to generate sounds, in this way.
Now, the operation of conventional acoustic exciter is described referring to FIG. 9. FIG. 9 shows an equivalent circuit diagram which represents the mechanical system of the acoustic exciter. In the circuit diagram, driving force Fva generated by magnetic circuit 221 and voice coil 228, and electromagnetic damping resistance Zea due to Fva are shown in a series circuit. Suspension 225's compliance Cs1a to magnetic circuit 221, suspension 225's mechanical resistance Rs1a to magnetic circuit 221, and mass Mma of magnetic circuit 221 and part of suspension 225 are shown in a series circuit. Also, mass Mf+v of vibrating section 227, voice coil 228, frame section 229 and part of suspension 225 is shown. Suspension 225's compliance Cs2a to vibrator 226, and suspension 225's mechanical resistance Rs2a to vibrator 226 are shown in a series circuit. Magnetic circuit 221's vibration speed Vma, vibrating section 227's vibration speed Va, and frame section 229's vibration speed Vfa are also shown.
As the equivalent circuit indicates, since vibrating section 227 for vibrating a vibration staff and frame section 229 share a unitized body their respective vibration speeds are the same, namely, Va=Vfa. Patent Document 1 provides an example of known technology information related to the present invention.
The vibration mass of the above conventional acoustic exciter includes that of vibrator 226 consisting of vibrating section 227, voice coil 228 and frame section 229, and that of part of suspension 225. The vibration mass remains constant regardless of the frequency. Therefore, although it provides a substantial vibration by series resonance at the lowest resonance frequency F0, the vibration decreases in other frequency region because energy is consumed by the load of the entire vibration mass. Loss due to the loading mass reveals its significance in the high frequency region; so is attenuation with the vibration. As the result, many of the conventional acoustic exciters demonstrate low operating efficiency, narrow sound reproduction range. There are problems in this sector still left to be solved; viz. the sound pressure and the quality of reproduced sounds.
Patent Document 1:                Japanese Patent Unexamined Publication No. S61-21699        